Some people desire to enjoy a physical sensation of a music in addition to listening to the music through the ear. A physical sensation of the music may be obtained by boosting bass tones to a higher loudness level. In the conventional practice of boosting bass tones, an equalizer is used to boost the bass region of an audio signal, and the boosted audio signal is amplified by a high capacity output amplifier, an amplified output signal of which is used to drive a gigantic woofer (loudspeaker used devotedly for the bass region). However, this approach fails to yield the intended effect unless the boosting of the bass tones takes place to a great extent. If it is attempted to achieve a similar effect with a low capacity output amplifier and a small size loudspeaker, there results a distortion of sounds.
It is recognized that auditory sensation of a man is susceptible to a tendency that he feels bass tones boosted when listening to harmonics containing a lot of overtones of bass components. There is a proposal that exploits this tendency to achieve an apparent boosting of bass tones by feeding bass components of an input audio signal to a nonlinear circuit to produce overtones of bass components in the input audio signal, which are then added to the input audio signal.
To give an example, Japanese Laid-Open Patent Application No. 328,481/1993 proposes a technique illustrated in FIG. 1. Specifically, stereophonic left-channel and right-channel signals from input terminals 11L and 11R are passed through low pass filters 12L and 12R, respectively, having a cut-off frequency of 100 Hz to pick out bass components equal to or less than 100 Hz, which are then subject to full wave rectification in a full wave rectifier circuit 13. An output signal from the full wave rectifier 13 is then passed through a bandpass filter 14 having a pass band of 100-200 Hz. In other words, double overtone signals of bass components produced by the full wave rectifier circuit 13 may be obtained from the bandpass filter 14 and added to the left-channel and right-channel signals from the input terminals 11L and 11R to be delivered to output terminals 15L and 15R.
The prior art shown in FIG. 1 picks out bass components through the low pass filters 12L, 12R. These low pass filters 12L, 12R have a cut-off frequency of 100 Hz, and have an increased time constant, which causes the output signal from the bandpass filter 14 to be synthesized in a considerably lagging relationship with respect to the input signals from the input terminals 11L, 11R. This means that signals in the input audio signal which represent vocals and tones from alto or middle frequency range musical instruments such as tenor saxophone as well as tones from high pitch or frequency range musical instruments such as a violin and a flute would be displaced in time with respect to signals representing tones from bass instruments such as a base and a bass drum, presenting a strange sensation for a concurrent rendition of these musical instruments.
Also proposed in Japanese Laid-Open Patent Application No. 186,008/1989 is a technique illustrated in FIG. 2. An input audio signal from an input terminal 11 is fed to a low pass filter 12 having a cut-off frequency on the order of 100 Hz, and bass components from the low pass filter 12 are amplified by a power amplifier 16 before being input to a nonlinear circuit 17. The nonlinear circuit 17 comprises two diodes in anti-parallel connection which clip the positive and the negative side of the input signal amplitude, whereby the input signal waveform is distorted, producing harmonics components of the input signal or overtone signals. The overtones thus produced are added to the input audio signal from the input terminal 11 in a summer 18 to be delivered to an output terminal 15.
Again in this prior art, the low pass filter having a cut-off frequency on the order of 100 Hz is used, and thus them remains the problem of time misalignment between bass components and higher pitch components in the similar manner as occurs with the technique shown in FIG. 1. Furthermore, if a bass tone containing a component having a fundamental frequency of 110 Hz is concurrently input with a bass drum tone containing a component having a fundamental frequency of 100 Hz, the nonlinear circuit 17 would produce components representing both sum and difference between the both input signals, or 10 Hz component and 210 Hz component, resulting in boosting unwanted bass tones and producing sounds which are unmusical and granting.
In addition, proposed in Japanese Laid-Open Patent Application No. 295,178/1994 is a technique illustrated in FIG. 3. The technique is employed in a sound source unit for an electronic musical instrument. Accordingly, musical tone waveform data from an input terminal 1 comprises sinusoidal wave data having a fundamental frequency such as a sound produced by the oscillation of a single string, for example, and sinusoidal wave data for overtone frequencies thereof, and thus is not data which represent waveforms of musical tones from a plurality of musical instruments. This musical tone waveform data is delayed by one clock period (a sampling period of the musical tone waveform data) in an interruption circuit 21a in a differential circuit 21. The delayed data is subtracted from non-delayed data in a subtractor 21b, and a result of subtraction is delivered as a differential data to be input to a non-linear conversion table 22 where the differential data is subject to a non-linear conversion by the non-linear conversion table 22. The converted data is summed with multiplication output data from a multiplier 23a in an additive circuit 23. A result of summation is delivered as musical tone data having added bass tones to an output terminal 15. The output musical tone data is also delayed by one clock period by a delay circuit 23c to be input to the multiplier 23a, which multiplies the input data by a diversion preventive coefficient a.
In this manner, higher pitch components are boosted in the differential circuit 21 so that the non-linear conversion by the non-linear conversion table 22 allows musical tones, which are overtones based on the higher pitch components, to be produced, while bass components are boosted in the additive circuit 23, with the distortion of overtones and the like which are based on the higher pitch components being boosted to a greater degree than the distortion of overtones and the like which are based on the bass components while allowing the bass components to be also boosted. Descriptions are given that the differential circuit 21 exhibits a response similar to a high pass filter with respect to input waveform data, that bass components similar to waveform data which is input appear in the additive circuit 23, and that a musical tone waveform signal is obtained which includes a distortion of overtones and the like resulting from the higher pitch components.
In this prior art, as many as six versions of input-output response of the non-linear function which the non-linear conversion table 22 yields are indicated in FIG. 5 of the above cited Application. These responses have a point symmetry as illustrated in FIG. 4 with respect to an input reference point, namely, a point of intersection P0 between an input axis and an output axis, as illustrated. As a consequence, overtones which are produced as a result of the non-linear conversion by the non-linear conversion table 22 include more of odd-numbered harmonics (overtones) than even-numbered harmonics (overtones), and it is noted that such a musical tone having an abundance of odd-numbered overtones disadvantageously leads to an indistinct and vacant timbre. As described, the higher pitch components of the input musical tone data is boosted in the differential circuit 21 before it is fed to the non-linear conversion table 22. In other words, all of higher pitch components in the input musical tone data are boosted before being fed to the non-linear conversion table 22. Hence, where the input data comprises an output signal from a CD (compact disc) player or a rendition output signal from an electronic musical instrument, not only musical tone data from a bass musical instrument such as a base or a bass drum, but data for tones from a variety of musical instruments such as data for vocals and tones from alto musical instrument such as saxophone as well as data for musical tones from alto and higher pitch musical instruments such as violin, flute and the like are also included; or more specifically, what is input to the table 22 has an abundance of data representing musical tones having components equal to or higher than 400 Hz. Accordingly, data for musical tones in the alto and higher pitch range, which are unnecessary for boosting musical tone data for bass tones, are also boosted and subjected to the non-linear conversion by the non-linear conversion table 22, causing higher pitch data to be produced which are distorted in an unnecessary manner. This, in addition to degrading the boosting of bass tones in a relative sense, results in a cross modulation when the plurality of varieties of alto and higher pitch musical tone data are input to the non-linear conversion table, producing components corresponding to differences and sums of frequencies among the plurality of alto and higher pitch musical tone data, that is, components which are not originally contained in the musical tone signal, and thus disadvantageously producing acoustic abnormalities in the auditory sensation.
It is an object of the present invention to provide an apparatus and a method for acoustic effect which is capable of producing overtones of fundamental tones of bass musical instruments such as a base, a bass drum and the like and boosting bass tones while maintaining a time concurrency between bass components and alto and/or higher pitch components so as to yield brilliant and clearly intonated sounds.
It is another object of the invention to provide an apparatus and a method for acoustic effect which is capable of producing overtones of fundamental tones of bass musical instruments such as a bass, a bass drum and the like and boosting bass tones while maintaining a time concurrency between bass components and alto and/or higher pitch components without producing components which are not originally present in the musical tone signal to cause acoustic abnormalities in the auditory sensation.
It is to be understood that what is generically referred to herein as a bass musical instrument is one which produces a fundamental tone equal to or below 200 Hz. Accordingly, while it is possible to produce a fundamental tone of 300 Hz with the bass, the latter is not included in the bass musical instrument when it is used to produce a fundamental tone of such a higher pitch.